The October 2014 issue of Astronomy Now is on sale! Order direct from our store (free 1st class post & to UK addresses). Astronomy Now is the only astronomy magazine specially designed to be read on tablets and phones. Download the app from Google Play Store or the Apple App Store.

Top Stories

Earthshine used to test life detection method
...By imagining the Earth as an exoplanet, scientists observing our planet's reflected light on the Moon with ESO's Very Large Telescope have demonstrated a way to detect life on other worlds...READ MORESolid buckyballs discovered in space
...Astronomers using NASA’s Spitzer Space Telescope have detected a particular type of molecule, given the nickname “buckyball”, in a solid form for the first time...READ MORESteamy water-world gets the Hubble treatment
...Hubble Space Telescope observations of a 7 Earth-mass planet find an unusual water-rich world swathed in a thick, steamy atmosphere...READ MORE

Record low for brown dwarfKEITH COOPERASTRONOMY NOWPosted: 01 February 2010

A bizarre failed star with a record-breaking low surface temperature has been discovered by the United Kingdom Infrared Telescope (UKIRT) in Hawaii, extending the range of extremes the properties of these kinds of objects can possess.

Discovered by Dr Ben Burningham of the University of Hertfordshire, the sub-stellar object – termed a brown dwarf – is called SDSS 1416+13B. Existing in a binary system with another brown dwarf, the similarly named SDSS 1416+13A, it has some puzzling features. For instance, it is so dim and cool that it can only be seen in infrared light, whereas its larger, warmer companion can be observed in visible light. Furthermore, when the Japanese 8.2-metre Subaru Telescope on Hawaii observed it in near-infrared wavelengths, it appeared bluer than any brown dwarf ever seen before. This has been attributed to copious amounts of methane in its atmosphere, which causes large gaps to appear in its spectrum.

The near-infrared image of the brown dwarfs on the left, in which brown dwarf B appears very blue, and the mid-infrared image on the right, in which brown dwarf B appears very blue. Image: JAC/UKIRT/Spitzer Space Telescope/University of Hertfordshire.

Things got weirder when NASA’s Spitzer Space Telescope observed it at mid-infrared wavelengths, and scientists found that it appeared redder than any other brown dwarf at those wavelengths. Typically, the colour of a brown dwarf at mid-infrared wavelengths is the best indicator of its surface temperature, and its extreme redness led to a temperature measurement of 227 degrees Celsius (500 kelvin) – the lowest temperature ever recorded for a brown dwarf. By comparison, brown dwarf A in the system has a surface temperature of 1,227 degrees Celsius (1,500 kelvin). The unusual colours are being enhanced by the gravity of the brown dwarfs, and from these enhancements their masses can be estimated: brown dwarf B has an estimated mass of about 30 times that of Jupiter (one Jupiter mass is 1.9 x 1027 kilograms), while its partner is 75 times the mass of Jupiter.

Brown dwarfs are a link between stars and planets. The distinction between a gas giant planet and a brown dwarf can be a little blurred, but the difference between stars and brown dwarfs is clear. Whereas stars generate energy from nuclear fusion instigated by the high pressures and temperatures within their cores, brown dwarfs are not massive enough to ignite nuclear fusion. Instead they generate their meagre energy by gradual gravitational contraction.

The near-infrared spectrum of brown dwarf B shows a large gap in the spectrum between 1.7 and 2.5 microns where there is barely any light. This makes the brown dwarf appear blue. Image: Subaru Telescope (NAOJ)/University of Hertfordshire.

This contraction happens quickest in the first billion years of a brown dwarf’s life, after which the brown dwarf begins to cool, and the more massive the brown dwarf is the slower it cools. Brown dwarf B is a moderately massive object (officially the lower limit for a brown dwarf is 13 Jupiter masses, but this is hotly debated, while brown dwarf A is almost, but not quite, massive enough to be a fully-fledged star) so for it to be so cool it must be very old – about ten billion years old. This is backed up by the fact that both objects are lacking in heavy elements, a characteristic of very old stars that date from a time when there wasn’t the same abundance of heavy elements that there is today.

“This is the fourth time in three years that UKIRT has made a record-breaking discovery of the coolest known brown dwarf, with an estimated temperature not far above 200 degrees Celsius,” says Dr Philip Lucas of the University of Hertfordshire. “Its colours are so extreme that this object will keep a lot of physicists busy trying to explain it.”

Indeed, it is quite likely that there are older, even cooler brown dwarfs out there somewhere, so it’s a good bet that there are still some more record-breaking discoveries to be made.

The PlanetsFrom tiny Mercury to distant Neptune and Pluto, The Planets profiles each of the Solar System's members in depth, featuring the latest imagery from space missions. The tallest mountains, the deepest canyons, the strongest winds, raging atmospheric storms, terrain studded with craters and vast worlds of ice are just some of the sights you'll see on this 100-page tour of the planets. GET YOUR COPY

Hubble RebornHubble Reborn takes the reader on a journey through the Universe with spectacular full-colour pictures of galaxies, nebulae, planets and stars as seen through Hubble's eyes, along the way telling the dramatic story of the space telescope, including interviews with key scientists and astronauts. GET YOUR COPY

3D UniverseWitness the most awesome sights of the Universe as they were meant to be seen in this 100-page extravaganza of planets, galaxies and star-scapes, all in 3D! GET YOUR COPY